Add string overloads to DbDataReader.Get*()

[roji]

See https://github.com/dotnet/corefx/issues/31595

This is the 3rd and last in this batch of System.Data changes.

Note that for the naming and namespace for the new DataReaderExtensions class, see dotnet/corefx#36123 (comment) and dotnet/corefx#36123 (comment).

[roji]

/cc @divega @terrajobst

[terrajobst]

Tagging @agocke

[terrajobst]

@roji

Please resolve the merge conflicts by doing this:

git checkout master
git pull
git checkout DbDataReaderStringOverloads
git rebase master
git reset src/netstandard/src/ApiCompatBaseline.*
git checkout src/netstandard/src/ApiCompatBaseline.*
.\build.cmd
git add .
git rebase --continue
git push origin DbDataReaderStringOverloads -f

[wtgodbe]

Approved for preview7